
// This defines a set of argument wrappers and related factory methods that
// can be used specify the refcounting and reference semantics of arguments
// that are bound by the Bind() function in base/bind.h.
//
// The public functions are base::Unretained() and base::ConstRef().
// Unretained() allows Bind() to bind a non-refcounted class.
// ConstRef() allows binding a constant reference to an argument rather
// than a copy.
//
//
// EXAMPLE OF Unretained():
//
//   class Foo {
//    public:
//     void func() { cout << "Foo:f" << endl;
//   };
//
//   // In some function somewhere.
//   Foo foo;
//   Callback<void(void)> foo_callback =
//       Bind(&Foo::func, Unretained(&foo));
//   foo_callback.Run();  // Prints "Foo:f".
//
// Without the Unretained() wrapper on |&foo|, the above call would fail
// to compile because Foo does not support the AddRef() and Release() methods.
//
//
// EXAMPLE OF ConstRef();
//   void foo(int arg) { cout << arg << endl }
//
//   int n = 1;
//   Callback<void(void)> no_ref = Bind(&foo, n);
//   Callback<void(void)> has_ref = Bind(&foo, ConstRef(n));
//
//   no_ref.Run();  // Prints "1"
//   has_ref.Run();  // Prints "1"
//
//   n = 2;
//   no_ref.Run();  // Prints "1"
//   has_ref.Run();  // Prints "2"
//
// Note that because ConstRef() takes a reference on |n|, |n| must outlive all
// its bound callbacks.
//

#ifndef __base_bind_helpers_h__
#define __base_bind_helpers_h__

#pragma once

#include "base/baseTemplateUtil.h"

namespace base
{
	namespace internal
	{

		// Use the Substitution Failure Is Not An Error (SFINAE) trick to inspect T
		// for the existence of AddRef() and Release() functions of the correct
		// signature.
		//
		// http://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error
		// http://stackoverflow.com/questions/257288/is-it-possible-to-write-a-c-template-to-check-for-a-functions-existence
		// http://stackoverflow.com/questions/4358584/sfinae-approach-comparison
		// http://stackoverflow.com/questions/1966362/sfinae-to-check-for-inherited-member-functions
		//
		// The last link in particular show the method used below.
		//
		// For SFINAE to work with inherited methods, we need to pull some extra tricks
		// with multiple inheritance.  In the more standard formulation, the overloads
		// of Check would be:
		//
		//   template <typename C>
		//   Yes NotTheCheckWeWant(Helper<&C::TargetFunc>*);
		//
		//   template <typename C>
		//   No NotTheCheckWeWant(...);
		//
		//   static const bool value = sizeof(NotTheCheckWeWant<T>(0)) == sizeof(Yes);
		//
		// The problem here is that template resolution will not match
		// C::TargetFunc if TargetFunc does not exist directly in C.  That is, if
		// TargetFunc in inherited from an ancestor, &C::TargetFunc will not match,
		// |value| will be false.  This formulation only checks for whether or
		// not TargetFunc exist directly in the class being introspected.
		//
		// To get around this, we play a dirty trick with multiple inheritance.
		// First, We create a class BaseMixin that declares each function that we
		// want to probe for.  Then we create a class Base that inherits from both T
		// (the class we wish to probe) and BaseMixin.  Note that the function
		// signature in BaseMixin does not need to match the signature of the function
		// we are probing for; thus it's easiest to just use void(void).
		//
		// Now, if TargetFunc exists somewhere in T, then &Base::TargetFunc has an
		// ambiguous resolution between BaseMixin and T.  This lets us write the
		// following:
		//
		//   template <typename C>
		//   No GoodCheck(Helper<&C::TargetFunc>*);
		//
		//   template <typename C>
		//   Yes GoodCheck(...);
		//
		//   static const bool value = sizeof(GoodCheck<Base>(0)) == sizeof(Yes);
		//
		// Notice here that the variadic version of GoodCheck() returns Yes here
		// instead of No like the previous one. Also notice that we calculate |value|
		// by specializing GoodCheck() on Base instead of T.
		//
		// We've reversed the roles of the variadic, and Helper overloads.
		// GoodCheck(Helper<&C::TargetFunc>*), when C = Base, fails to be a valid
		// substitution if T::TargetFunc exists. Thus GoodCheck<Base>(0) will resolve
		// to the variadic version if T has TargetFunc.  If T::TargetFunc does not
		// exist, then &C::TargetFunc is not ambiguous, and the overload resolution
		// will prefer GoodCheck(Helper<&C::TargetFunc>*).
		//
		// This method of SFINAE will correctly probe for inherited names, but it cannot
		// typecheck those names.  It's still a good enough sanity check though.
		//
		// Works on gcc-4.2, gcc-4.4, and Visual Studio 2008.
		//
		// TODO(ajwong): Move to ref_counted.h or template_util.h when we've vetted
		// this works well.
		template<typename T>
		class SupportsAddRefAndRelease
		{
			typedef char Yes[1];
			typedef char No[2];

			struct BaseMixin
			{
				void AddRef();
				void Release();
			};

			// MSVC warns when you try to use Base if T has a private destructor, the
			// common pattern for refcounted types. It does this even though no attempt to
			// instantiate Base is made.  We disable the warning for this definition.
#pragma warning(disable:4624)
			struct Base : public T, public BaseMixin {};
#pragma warning(default:4624)

			template<void(BaseMixin::*)(void)>  struct Helper {};

			template<typename C>
			static No& Check(Helper<&C::AddRef>*, Helper<&C::Release>*);

			template<typename >
			static Yes& Check(...);

		public:
			static const bool value = sizeof(Check<Base>(0,0)) == sizeof(Yes);
		};


		// Helpers to assert that arguments of a recounted type are bound with a
		// scoped_refptr.
		template<bool IsClasstype, typename T>
		struct UnsafeBindtoRefCountedArgHelper : false_type {};

		template<typename T>
		struct UnsafeBindtoRefCountedArgHelper<true, T>
			: integral_constant<bool, SupportsAddRefAndRelease<T>::value> {};

		template<typename T>
		struct UnsafeBindtoRefCountedArg : false_type {};

		template<typename T>
		struct UnsafeBindtoRefCountedArg<T*>
			: UnsafeBindtoRefCountedArgHelper<is_class<T>::value, T> {};


		template<typename T>
		class UnretainedWrapper
		{
		public:
			explicit UnretainedWrapper(T* o) : obj_(o) {}
			T* get() { return obj_; }
		private:
			T* obj_;
		};

		template<typename T>
		class ConstRefWrapper
		{
		public:
			explicit ConstRefWrapper(const T& o) : ptr_(&o) {}
			const T& get() { return *ptr_; }
		private:
			const T* ptr_;
		};


		// Unwrap the stored parameters for the wrappers above.
		template<typename T>
		T Unwrap(T o) { return o; }

		template<typename T>
		T* Unwrap(UnretainedWrapper<T> unretained) { return unretained.get(); }

		template<typename T>
		const T& Unwrap(ConstRefWrapper<T> const_ref)
		{
			return const_ref.get();
		}


		// Utility for handling different refcounting semantics in the Bind()
		// function.
		template<typename ref, typename T>
		struct MaybeRefcount;

		template<typename T>
		struct MaybeRefcount<base::false_type, T>
		{
			static void AddRef(const T&) {}
			static void Release(const T&) {}
		};

		template<typename T, size_t n>
		struct MaybeRefcount<base::false_type, T[n]>
		{
			static void AddRef(const T*) {}
			static void Release(const T*) {}
		};

		template<typename T>
		struct MaybeRefcount<base::true_type, UnretainedWrapper<T> >
		{
			static void AddRef(const UnretainedWrapper<T>&) {}
			static void Release(const UnretainedWrapper<T>&) {}
		};

		template<typename T>
		struct MaybeRefcount<base::true_type, T*>
		{
			static void AddRef(T* o) { o->AddRef(); }
			static void Release(T* o) { o->Release(); }
		};

		template<typename T>
		struct MaybeRefcount<base::true_type, const T*>
		{
			static void AddRef(const T* o) { o->AddRef(); }
			static void Release(const T* o) { o->Release(); }
		};

	} //namespace internal

	template<typename T>
	inline internal::UnretainedWrapper<T> Unretained(T* o)
	{
		return internal::UnretainedWrapper<T>(o);
	}

	template<typename T>
	inline internal::ConstRefWrapper<T> ConstRef(const T& o)
	{
		return internal::ConstRefWrapper<T>(o);
	}

} //namespace base

#endif //__base_bind_helpers_h__